He is an environmental lawyer, co-founder and senior attorney of the Natural Resources Defense Council (1970-1977) and founding president of the World Resources Institute (1982-1993).
He is currently a professor at the Vermont Law School, Senior Fellow at The Tellus Institute and Demos,  and Co-Chair and Senior Fellow of The Democracy Collaborative’s  Next System Project.

His other positions have included;

• Chair of the US Council on Environmental Quality (1979-1981)
• Administrator of the United Nations Development Programme (1993-1999)
• Chair of the UN Development Group
• Dean of the Yale School of Forestry and Environmental Studies (1999-2009).

He is author or co-author of ;

• Red Sky at Morning: America and the Crisis of the Global Environment (2004)
• Global Environmental Governance (2006)
• The Bridge at the Edge of the World: Capitalism, the Environment and Crossing from Crisis to Sustainability (2008)
• America the Possible: Manifesto for a New Economy (2012), and Angels by the River: A Memoir (2014).

Speth has received many honorary degrees, and awards such as the Blue Planet Prize (2002).